1. Field of the Invention
The present invention relates to a spring apparatus adapted for use in, for example, a valve train or the like of an internal combustion engine.
2. Description of the Related Art
A valve train of an internal combustion engine comprises an intake valve, an exhaust valve, a valve spring (compression coil spring) for urging these valves in their closing direction, a cam mechanism for driving the valves in their opening direction, etc. The intake valve serves to open and close an intake port in a cylinder head. The exhaust valve serves to open and close an exhaust port in the cylinder head. An installation load and a maximum load of the valve spring are set to appropriate values to prevent the valves jumping and bouncing when the engine is operated at high-speed rotation. At the time of low-speed rotation, the force of inertia of the valve train is lower than at the time of high-speed rotation. During low-speed rotation, therefore, the load on the valve spring may be smaller than during high-speed rotation. If the valve spring used is a compression coil spring, however, the spring load cannot be changed according to the speed of rotation, high or low.
A gas spring is used in a valve train described in Jpn. Pat. Appln. KOKAI Publication No. 10-288012 (Prior Art 1), for example. Use of the gas spring enables the spring load to be changed, depending on the speed of rotation. This valve train comprises a gas pressure adjusting mechanism for changing the pressure of the gas spring. The pressure of the gas spring at the time of low-speed rotation is made lower than the spring pressure at the time of high-speed rotation. By doing this, the spring load can be lowered to reduce a friction loss during low-speed rotation.
On the other hand, a valve train described in Jpn. Pat. Appln. KOKAI Publication No. 58-217711 (Prior Art 2) comprises a compression coil spring and a hydraulic chamber. The hydraulic chamber has a function to switch the height of a spring seat for the compression coil spring. This valve train changes the amount of compression by varying the capacity of the hydraulic chamber to change the height of the spring seat.
The valve train of Prior Art 1 requires use of a gas pressure adjusting mechanism, a pressure supply source, etc. for changing the pressure of the gas spring. The gas pressure adjusting mechanism is composed of a solenoid changeover valve and pipes. Therefore, this valve train is complicated in structure and has a high cost. Further, changing the gas pressure requires external power and energy, possibly exerting a bad influence on the fuel consumption.
The valve train of Prior Art 2 changes the pressure in the hydraulic chamber in accordance with the speed of rotation of the internal combustion engine. Therefore, it requires use of a hydraulic mechanism, a hydraulic source, etc. for changing the pressure in the hydraulic chamber. The hydraulic mechanism of this type is also composed of a solenoid changeover valve and pipes. Thus, the valve train is complicated in structure and has a high cost. Further, there is a problem that energy is needed to change the hydraulic pressure.